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<title>Conditions (Debugging with GDB)</title>
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Breakpoints.html#Breakpoints" rel="up" title="Breakpoints">
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<a name="Conditions"></a>
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<div class="header">
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<p>
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Next: <a href="Break-Commands.html#Break-Commands" accesskey="n" rel="next">Break Commands</a>, Previous: <a href="Disabling.html#Disabling" accesskey="p" rel="prev">Disabling</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Break-Conditions"></a>
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<h4 class="subsection">5.1.6 Break Conditions</h4>
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<a name="index-conditional-breakpoints"></a>
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<a name="index-breakpoint-conditions"></a>
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<p>The simplest sort of breakpoint breaks every time your program reaches a
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specified place. You can also specify a <em>condition</em> for a
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breakpoint. A condition is just a Boolean expression in your
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programming language (see <a href="Expressions.html#Expressions">Expressions</a>). A breakpoint with
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a condition evaluates the expression each time your program reaches it,
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and your program stops only if the condition is <em>true</em>.
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</p>
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<p>This is the converse of using assertions for program validation; in that
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situation, you want to stop when the assertion is violated—that is,
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when the condition is false. In C, if you want to test an assertion expressed
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by the condition <var>assert</var>, you should set the condition
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‘<samp>! <var>assert</var></samp>’ on the appropriate breakpoint.
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</p>
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<p>Conditions are also accepted for watchpoints; you may not need them,
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since a watchpoint is inspecting the value of an expression anyhow—but
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it might be simpler, say, to just set a watchpoint on a variable name,
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and specify a condition that tests whether the new value is an interesting
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one.
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</p>
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<p>Break conditions can have side effects, and may even call functions in
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your program. This can be useful, for example, to activate functions
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that log program progress, or to use your own print functions to
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format special data structures. The effects are completely predictable
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unless there is another enabled breakpoint at the same address. (In
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that case, <small>GDB</small> might see the other breakpoint first and stop your
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program without checking the condition of this one.) Note that
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breakpoint commands are usually more convenient and flexible than break
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conditions for the
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purpose of performing side effects when a breakpoint is reached
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(see <a href="Break-Commands.html#Break-Commands">Breakpoint Command Lists</a>).
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</p>
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<p>Breakpoint conditions can also be evaluated on the target’s side if
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the target supports it. Instead of evaluating the conditions locally,
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<small>GDB</small> encodes the expression into an agent expression
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(see <a href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</a>) suitable for execution on the target,
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independently of <small>GDB</small>. Global variables become raw memory
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locations, locals become stack accesses, and so forth.
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</p>
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<p>In this case, <small>GDB</small> will only be notified of a breakpoint trigger
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when its condition evaluates to true. This mechanism may provide faster
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response times depending on the performance characteristics of the target
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since it does not need to keep <small>GDB</small> informed about
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every breakpoint trigger, even those with false conditions.
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</p>
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<p>Break conditions can be specified when a breakpoint is set, by using
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‘<samp>if</samp>’ in the arguments to the <code>break</code> command. See <a href="Set-Breaks.html#Set-Breaks">Setting Breakpoints</a>. They can also be changed at any time
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with the <code>condition</code> command.
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</p>
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<p>You can also use the <code>if</code> keyword with the <code>watch</code> command.
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The <code>catch</code> command does not recognize the <code>if</code> keyword;
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<code>condition</code> is the only way to impose a further condition on a
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catchpoint.
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</p>
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<dl compact="compact">
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<dd><a name="index-condition"></a>
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</dd>
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<dt><code>condition <var>bnum</var> <var>expression</var></code></dt>
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<dd><p>Specify <var>expression</var> as the break condition for breakpoint,
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watchpoint, or catchpoint number <var>bnum</var>. After you set a condition,
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breakpoint <var>bnum</var> stops your program only if the value of
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<var>expression</var> is true (nonzero, in C). When you use
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<code>condition</code>, <small>GDB</small> checks <var>expression</var> immediately for
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syntactic correctness, and to determine whether symbols in it have
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referents in the context of your breakpoint. If <var>expression</var> uses
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symbols not referenced in the context of the breakpoint, <small>GDB</small>
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prints an error message:
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</p>
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<div class="smallexample">
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<pre class="smallexample">No symbol "foo" in current context.
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</pre></div>
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<p><small>GDB</small> does
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not actually evaluate <var>expression</var> at the time the <code>condition</code>
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command (or a command that sets a breakpoint with a condition, like
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<code>break if …</code>) is given, however. See <a href="Expressions.html#Expressions">Expressions</a>.
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</p>
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</dd>
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<dt><code>condition <var>bnum</var></code></dt>
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<dd><p>Remove the condition from breakpoint number <var>bnum</var>. It becomes
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an ordinary unconditional breakpoint.
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</p></dd>
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</dl>
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<a name="index-ignore-count-_0028of-breakpoint_0029"></a>
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<p>A special case of a breakpoint condition is to stop only when the
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breakpoint has been reached a certain number of times. This is so
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useful that there is a special way to do it, using the <em>ignore
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count</em> of the breakpoint. Every breakpoint has an ignore count, which
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is an integer. Most of the time, the ignore count is zero, and
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therefore has no effect. But if your program reaches a breakpoint whose
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ignore count is positive, then instead of stopping, it just decrements
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the ignore count by one and continues. As a result, if the ignore count
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value is <var>n</var>, the breakpoint does not stop the next <var>n</var> times
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your program reaches it.
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</p>
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<dl compact="compact">
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<dd><a name="index-ignore"></a>
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</dd>
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<dt><code>ignore <var>bnum</var> <var>count</var></code></dt>
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<dd><p>Set the ignore count of breakpoint number <var>bnum</var> to <var>count</var>.
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The next <var>count</var> times the breakpoint is reached, your program’s
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execution does not stop; other than to decrement the ignore count, <small>GDB</small>
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takes no action.
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</p>
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<p>To make the breakpoint stop the next time it is reached, specify
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a count of zero.
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</p>
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<p>When you use <code>continue</code> to resume execution of your program from a
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breakpoint, you can specify an ignore count directly as an argument to
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<code>continue</code>, rather than using <code>ignore</code>. See <a href="Continuing-and-Stepping.html#Continuing-and-Stepping">Continuing and Stepping</a>.
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</p>
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<p>If a breakpoint has a positive ignore count and a condition, the
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condition is not checked. Once the ignore count reaches zero,
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<small>GDB</small> resumes checking the condition.
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</p>
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<p>You could achieve the effect of the ignore count with a condition such
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as ‘<samp><span class="nolinebreak">$foo--</span> <= 0</samp>’<!-- /@w --> using a debugger convenience variable that
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is decremented each time. See <a href="Convenience-Vars.html#Convenience-Vars">Convenience
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Variables</a>.
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</p></dd>
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</dl>
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<p>Ignore counts apply to breakpoints, watchpoints, and catchpoints.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Break-Commands.html#Break-Commands" accesskey="n" rel="next">Break Commands</a>, Previous: <a href="Disabling.html#Disabling" accesskey="p" rel="prev">Disabling</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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